1. Field of the Invention
The present invention relates to a precision drilling method for drilling a straight hole in a work with a drill.
2. Description of the Prior Art
Generally, a drill having a small diameter is elastically bent by applying a relatively small stress to the drill. For example, when a contacting pressure, which results from contacting the drill rotating at a high speed with a work, is applied to the motor, there is a possibility of the top of the drill slipping on the work. In this case, as shown in FIG. 1A, the drill (D) is elastically bent so as to incline against the axis of a rotating device (R) for rotating the drill. When thus bent drill is fed toward the bottom of the work (W), a curved hole corresponding to the bent drill (D) will be formed in the work (W), as shown in FIG. 1B. Moreover, when a larger force than an elastic limit of the drill is applied to the drill, there is a possibility of causing a breakage of the drill.
Japanese Patent Early Publication [KOUKAI] No. 61-244404 describes about a method of drilling a deep hole in a printed circuit board with a drill. That is, in case of drilling the deep hole, the depth of which is 6 times or more of the diameter of the drill, in the printed circuit plate, a shallow hole is firstly drilled in the printed circuit board with a short drill, and then the deep hole is formed in the printed circuit board by drilling the shallow hole more deeply with a long drill. Since an amount of elastic deformation of the short drill is smaller than that of the long drill having the same diameter as the short drill, the shallow hole is easily drilled in the printed circuit board with the short drill without substantially causing the elastic deformation of the short drill. In addition, inasmuch as the top of the long drill is set in the shallow hole, the long drill may be smoothly fed into the printed circuit board to form the deep hole. However, in this prior art, after the shallow hole is drilled with the short drill, the long drill must be accurately transferred again to the position of the shallow hole. Therefore, for example, it is necessary for using an expensive X-Y table with a considerable precision.
On the other hand, Japanese Patent Early Publication [KOUKAI] No. 61-30310 describes about a drilling apparatus capable of preventing a breakage of the drill. As illustrated in FIGS. 17 and 18, the drilling apparatus comprises a rotatable spindle 11b supported in a spindle housing 15B, a driving device 12B for rotating the spindle 11B, a feed motor 23B for transferring the spindle housing 15B along the axial direction of the spindle 11B, a sensor 70B attached to a spindle housing 15B for monitoring a positional displacement of the spindle 11B along the axial direction of the spindle without physical contact, and a work-holding table(not shown), etc. An output signal from the sensor 70B is sent to an exchanging device 20B including an amplifier, so that an output signal corresponding to an axial force applied to the spindle is obtained from the exchanging device 20B. And then, the output signal from the exchanging device 20B is sent to a comparator 21B. In addition, an output signal corresponding to a critical value supplied from a setting device 22B is sent to the comparator 22B. In the comparator 22B, the output signal from the exchanging device 20B is compared with the output signal from the setting device 22B. When the output signal from the exchanging device 20B is smaller than that from the setting device 22B, that is, the axial force is smaller than the critical value, a drill 6B connected with the spindle 11B is stably fed toward the bottom of a work by the feed motor 23B. However, when the axial force is larger than the critical value, the drilling operation is stopped, the spindle housing 25B is transferred by the feed motor 23B until the drill is removed from the work, and then the drilling operation is started again to form a desired hole in the work. Therefore, since the axial force larger than the critical value is not applied to the drill 6B during the drilling operation, the breakage of the drill is effectively prevented.
Japanese Patent Early Publication [KOUKAI] No. 62-162405 describes another method of preventing the breakage of the drill. Since a rotatable spindle of a motor is connected with the drill through a magnetic joint portion, a phase shift between rotations of the spindle and the drill occurs at the magnetic joint portion in the drilling operation. That is, as a torque applied to the drill increases, the phase shift is larger. The torque applied to the drill can be obtained by detecting the phase shift. When the torque is larger than a critical value, the drilling operation is stopped, and then the drill is removed from the work. After scraps of the work are cleaned, the drilling operation is started again to form a desired hole in the work. In this prior art, since the torque larger than the critical value is not applied to the drill during the drilling operation, the breakage of the drill is effectively prevented.